In a number of industrial applications, it is either necessary or desirable to transfer heat from one fluid to another. This transfer is most commonly performed by a heat exchanger. Heat exchangers perform their function by using various flow designs and arrangements of different fluids, between which heat is transferred. These devices find use in refineries, fossil-fuel and nuclear power plants, in the chemical industry, air conditioning and refrigeration, as well as in cooling applications for small scale power devices. Oftentimes, they are given different names depending upon the environment in which they are used. Condensers, coolers, superheaters, evaporators, and other devices, are all properly considered heat exchangers.
One prominent application of heat exchangers is in distillation processes. These processes involve the separation of multi-component mixtures into purified fractions by one or more cycles of vaporization and condensation. Because the vaporized and condensed states are of different internal energies, heat transfer to and from these states is the most common means used for their interconversion.
In the petroleum industry, an interconversion method known as fractional or differential distillation is used. This technique is important for the separation of multi-component liquids in which the individual liquids have boiling points that lie close together. This technique involves repeated vaporization and condensation. Heat exchangers are of particular importance here. Due to the multi-component nature of the distillate and its multiple boiling points, the heat exchanger must be efficient enough to allow the closest possible thermal contact between the rising vapor and the descending condensate. Thus, efficient, trouble-free heat exchangers have particular applicability for use in connection with the refining of petroleum products and in the chemical and petrochemical industries.
Heat exchangers operate most efficiently where the surface area of the heat exchanger that is in contact with the medium to which heat is transferred is maximized. One way to achieve improved heat transfer is to minimize any contaminants located on the heat transfer surfaces. When heat exchange surfaces are deposited with particulates and other matter, thereby decreasing the effective area of the heat transfer surfaces, they are “fouled.” Such fouling will adversely affect the heat exchange process and can potentially have a major negative impact on the economics of a refining operation. While minor fouling leads to a tolerable decrease in the efficiency of heat exchangers, this effect becomes more pronounced over time as the extent of fouling increases. In more extreme cases, fouling may lead to total failure of the heat exchanger with significant economic costs.
In petroleum refining applications, a common type of fouling is caused by particulate fouling. This may occur by the slow accumulation of foreign material, but is more likely to come from the breakdown of catalytic material used in the petroleum industry. Catalyst is often fabricated into small particles in which the active catalyst is immobilized onto a solid support with the goal of maximizing the catalyst surface area. Oftentimes, the catalyst is impregnated into the pores of a solid support. With time, the catalyst particles degrade and migrate into areas where they may foul equipment. The heat exchanger is one piece of equipment that is particularly susceptible to fouling due to the need to keep its surfaces contaminant-free. The small particle, high surface-area catalysts used in the refining industry are of the ideal configuration to cover the surface of the heat exchange element and decrease its efficiency. The most common scenario of particulate fouling by catalysts in petroleum refining is by the mechanisms of migration and attachment.
For example, Clarified Slurry Oil (CSO) heat exchangers are used in the field of petroleum refining in connection with fluid catalytic cracking units. They play an integral role in keeping the Fluid Catalytic Cracking Unit (FCCU) in heat balance and at maximum charge rate. The heat source of the main distillation column is the FCCU reactor effluent entering between the Light Cycle Oil (LCO) draw and the distillation tower bottom.
The CSO heat exchangers in a typical oil refining process are susceptible to fouling when the catalyst containment efficiency of the FCCU cyclone separators decreases. This event will usually occur near the end of run or when the integrity of the catalyst containment equipment is compromised. Fouling occurs when a layer of catalyst particles accumulates on the heat transfer surfaces of the heat exchanger. This layer of catalyst reduces the efficiency of the heat exchanger to transfer heat as a result of the loss of heat transfer surface area on the fouled surfaces.
One interim response to such a situation is to decrease the FCCU feed rate. This has the undesirable effect of decreasing product throughput.
The typical curative response to such a situation is to take the unit off-line and perform cleaning maintenance. While this will regenerate the apparatus to its earlier efficiencies, it is also an undesirable remedy. Whenever the FCCU is in turndown mode or is taken off-line for unscheduled maintenance, the refiner is losing the opportunity to operate the FCCU at its optimal revenue-generating capacity. Unplanned maintenance is much costlier than planned maintenance. Resources must be secured with little if any, lead time. The effect on unit operations costs of downstream units (e.g., wastewater treatment) will increase.
Moreover, cleaning the CSO heat exchangers is also a potentially dangerous task. A high pressure steam lance is typically used to remove the fouling layer. Liquid temperatures are approximately 700° F. The process is labor intensive and the liquids used have the potential to cause severe burns to personnel performing the operation. Any mechanism to eliminate or reduce the frequency of such maintenance will be beneficial to refining operations.
Because of the undesirable effects associated with fouled exchangers, there have been many attempts to address the problem.
For example, U.S. Pat. No. 4,370,236 teaches the purification of hydrocarbons by the electrostatic precipitation after the formation of an aqueous admixture of the hydrocarbon. The '236 purification method requires the precipitation of foulants. Additionally, the teaching of the '236 patent requires, among other things, the formation of an aqueous liquid.
In a somewhat related field, there have been efforts to address particulate fouling in gaseous streams. U.S. Pat. No. 5,318,102 to Spokoyny, et al., teaches how to improve resistance to fouling in heat exchangers for gas streams through the use of plate packs. Plate packs are a unique configuration of an array of heat transfer plates. This array affords a gradual decrease in pressure of the gas stream while maintaining good thermal contact with the gas and minimizing particulate deposition. This process only contemplates gaseous streams and fails to consider other fluids and, in particular, petroleum streams. U.S. Pat. No. 4,885,139 to Sparks, et al. also focuses on gaseous streams; in particular, on the removal of acidic gases from gaseous flow streams. Sparks also fails to consider fluids and requires a complex multistage process. Finally, U.S. Pat. No. 6,089,023 to Anderson et al. teaches a method that employs a decrease in temperature of the exhaust gas stream, thereby improving the efficiency of a subsequent electrostatic precipitation. Anderson, like Spokoyny and Sparks, is directed toward gaseous streams.
One common practice in the petroleum refining industry used to address the problem of foulants uses an arrangement of columns or chambers packed with ferrous metallic particles to which a charge is applied. The hydrocarbon fluid to be purified of contaminants is passed through the columns. While this method utilizes electric charge for purification, it is in actuality, a form of chromatography, as the contaminant particles are trapped by the column packing. The Gulftronics™ catalyst separator is currently the most common known commercial embodiment of this technology. It is a modular (skid mounted) apparatus designed to be placed in the CSO circulation loop. CSO enters the individual chambers of the Gulftronics™ which are aligned in series. Each chamber is filled with ferrous metallic spheres. An electronic charge is applied to these spheres to attract the catalyst particles to their surface, thus clarifying the CSO stream. Once the spheres accumulate enough catalyst particulates, the electric charge is turned off and the flow is redirected to new or regenerated purification chambers or back to the FCCU riser or to a slurry settler. When an adequate amount of time to purge the saturated chambers has elapsed, the flow may again be redirected to the original purification chambers which now contain reconditioned spheres. This method and apparatus entails complex hardware and requires intensive scheduled maintenance. The chromatographic columns must be regenerated or replaced as they become saturated. A non-chromatographic method which does not involve column regeneration or column switching is desirable. Moreover, space must be allocated for it immediately upstream of the CSO heat exchangers. Piping modifications must be made to tie the apparatus into the CSO circulation loop. Piping modifications must be made from the prior art apparatus into the FCCU riser or a slurry settler.
Because of the shortcomings of the prior art, the need presently exists for a simple in-line method to prevent or minimize fouling in these complex liquid streams.